I have noticed strange JIRA 5.1.2 behaviour. If I leave field 'time spent' empty on the resolution screen, JIRA does not show any validation messages and successfully resolves issue without logged work records. But when I put '0h' into , it says 'Time spent can not be set to zero' when I press 'Resolve' button even though leaving 'time spent' field empty is semantically equal to putting '0h' value there. Notice that there are no custom workflow validators for the 'Resolve' transition configured.
Is that an expected JIRA behaviour or is this a bug?